1. Field of the Invention
The present invention relates to two-component coating systems for producing flexible coatings. The coating systems comprise polyurethane prepolymers with allophanate structures and also amino-functional polyaspartic esters as curing agents.
2. Description of Related Art
Polyurethane-based or polyurea-based. two-component coating systems are known and used in the art. In general they include a liquid polyisocyanate component and a liquid, isocyanate-reactive component. Reaction of polyisocyanates with amines as an isocyanates-reactive component produces highly crosslinked polyurea coatings. Primary amines and isocyanates, however, usually react very rapidly with one another. Typical potlifes or gelling times often amount only to several seconds to a few minutes. Consequently these polyurea coatings cannot be applied manually, but only with special spraying apparatus. Such coatings, however, possess excellent physical properties.
One method from the literature of reducing the reactivity is to use prepolymers having a low NCO content. Through the use of NCO-functional prepolymers in combination with amines it is possible to produce flexible polyurea coatings.
U.S. Pat. No. 3,428,610 and U.S. Pat. No. 4,463,126 disclose the preparation of polyurethane/polyurea elastomers by curing NCO-functional prepolymers with aromatic diamines. These are preferably diprimary aromatic diamines, having at least one alkyl substituent of 2-3 carbon atoms positioned ortho to each amino group, and optionally also having methyl substituents in further positions ortho to the amino groups, such as, for example, diethyltolyldiamine (DETDA). U.S. Pat. No. 4,463,126 describes a process for producing solvent-free elastic coatings which involves curing NCO prepolymers based on isophorone diisocyanate (IPDI) and polyether polyols at room temperature using sterically hindered diprimary aromatic diamines.
A disadvantage associated with such systems is that the aromatic diamines have a propensity towards severe yellowing.
A further option for retarding the reaction between polyisocyanates and amines is to use secondary amines. EP-A 403 921 and U.S. Pat. No. 5,126,170 disclose the formation of polyurea coatings by reaction of polyaspartic esters with polyisocyanates. Polyaspartic esters possess low viscosity and reduced reactivity towards polyisocyanates and can therefore be used for preparing solvent-free coating compositions having extended potlifes. An additional advantage of polyaspartic esters is that the products are colorless.
Colorless aliphatic polyisocyanate prepolymers based on polyether polyols, however, cure extremely slowly with polyaspartic esters, and the coatings often possess a tacky surface.
It is an object of the present invention to provide two-component coating compositions for producing polyurea coatings, said compositions having potlives long enough to enable even manual application and permitting the production of flexible coatings which are both clear and colorless and have good performance data such as elasticity and hardness.
This object has now been achieved through the combination of specific allophanate polyisocyanates with polyaspartic esters.